Once surface water has been captured, transported and held in storage with minimal leakage, the next challenge is minimising evaporation and, where possible, flushing away salty water.
Minimise evaporation with windbreaks and surface covers
Pan evaporation is around 1600 mm per annum for the medium rainfall zone (see Figure 7).
This represents significant potential for the loss of water through evaporation from storage.
When water is evaporated from storage, salt is not evaporated with it. The salt is left behind, increasing the salinity of the remaining water. Even relatively fresh dams may turn saline as a result of excessive evaporation. Minimising the volume of evaporation from the dam will therefore help to limit the build up of salinity.
Figure 7 Average annual pan evaporation in south-west Western Australia (Laing et al. 1988).
Evaporation is a two phase process. Photons (sunlight) increase the surface temperature of a water body and may vaporise water from the dam surface. Moving air (wind) with a
moisture deficit then carries the water away. Establishing a windbreak, covering the surface
of the dam and shading the dam are three ways to interrupt this process and reduce evaporation.
Windbreaks
Trees established on the windward side of a dam can reduce wind velocity and the moisture deficit of air over the water (see Figure 8), and thereby reduce evaporation (Hipsey 2002).
Natural shelterbelts have been shown to reduce evaporation in dams by up to 36 per cent (Hipsey et al. 2004). The optimum distance between windbreak and dam to provide effective protection is dependent on the height of the trees. Figures range from a distance 4 to 6 times the windbreak height (Lantzke 1995) to 10 times (Department of Agriculture and Food 2005, Hipsey 2002). This suggests that evaporation control with windbreaks will be most effective where tall trees protect a dam with limited surface area, and is likely to be limited for dams with a very large surface area and where tree height is limited (by climate or soil).
Figure 8 Dam windbreak system (Source: Hipsey 2002).
a. The quiet and wake zones created downwind.
b. Wind speed reduction downwind compared with upwind.
c. Relative moisture levels downwind of the windbreak showing an increase in the quiet zone and a decrease further downwind.
Species with non-invasive roots should be used to avoid roots breaching the dam wall, possibly causing the dam to leak. Hipsey (2002) recommends a minimum buffer of two tree heights between the dam wall and the trees to limit the chance of tree roots compromising the integrity of the dam wall.
Artificial windbreaks have been used to limit the evaporation of water from soils (Lynch et al.
1980). They also have the potential to reduce evaporation from farm water supplies. Artificial windbreaks do not have the drawbacks associated with roots and they have the added
benefit of being able to be erected on the dam wall, taking advantage of the extra height created by the wall itself. The disadvantages include cost, particularly if the structure is built to withstand strong winds, and maintenance over the medium to long term.
Shade cloth is one material that shows some promise (Lantzke, pers. comm.), both as a material for forming an artificial windbreak, and as a suspended cover. Hipsey et al. (2002) showed that evaporation could be reduced by approximately 20 per cent with a 2.5 m high shade cloth barrier at two sites in Katanning and Corrigin, Western Australia.
Surface covers and shades
Covering the dam surface prevents exposure to wind and sun, and has proven to be the most efficient way of limiting evaporation from water bodies. At $2–$3/m2 it is relatively cheap in comparison to other covering materials such as corrugated iron and pool blankets. Shade cloth suspended over a dam on a frame provides shade and protection from the wind. The frame may be engineered to allow stock access to the water if required and for dam maintenance. Shading has an added bonus in helping to prevent water quality problems associated with algal blooms. The cooler temperatures caused by the shading of the dam may also provide aquaculture benefits, particularly where water temperatures rise above optimum levels for production during summer months.
Swimming pool ‘blankets’ are a well established technology and have resulted in significant reductions in water consumption in houses with swimming pools in residential areas of Western Australia, and similar reductions would be expected in dams. These covers are expensive, however, at a cost of $50 000–$115 000 per hectare for a product with a 3–12 year guarantee, and may not be suitable for on-farm water supplies unless the water is being used for high value production.
One major issue with using pool covers on dams is the lack of a pathway by which rain falling on the cover can enter the dam (Alan Lieback, pers. comm.). Another is the lack of oxygen transfer in the water, which may cause anaerobic conditions to form under the blanket (Department of Natural Resources and Mines 2003). This has the potential to cause problems if the dam water is rich in dissolved iron and the water is to be piped or used for irrigation. As the water in the reticulation is exposed to oxygen, iron bacteria are likely to cause pipe blockages. The lack of oxygen transfer in the water may also kill plants and animals in the dam, and this has severe consequences if the dam has aquaculture production or habitat value. On the other hand, an advantage—the result primarily of blocking sunlight but also reduced stock access—is the reduction in risk of toxic algal blooms.
If dams have fluctuating water levels the covers may need to be adjusted. If the dam empties entirely, the cover should be temporarily removed to prevent it inadvertently becoming a poly lining as the dam fills again. Covers may also prevent stock access, although this problem can be resolved by establishing troughs.
Companies such as Jaylon Industries (Malaga) offer a product called floating blankets (see Figure 9), similar to pool blankets, which lasts 5–7 years at a cost of $6–$8/m2 or $60 000 to
$80 000/hectare (Alan Liebeck pers. comm.).
Figure 9 Floating covers on a dam (Photo: Alan Liebeck).
E-VapCap is another product similar to the pool blanket, produced in the eastern states by a joint venture of Sealed Air Australia Pty Ltd, Evaporation Control Systems (ECS) Pty Ltd and Darling Downs Tarpaulins Pty Ltd. The product is a multi-layered polyethylene (PE)
membrane approximately 0.5 mm thick containing buoyancy cells trapped within the layers.
The E-VapCap cover costs $60 000 per hectare, and has been proven at close to 100 per cent efficiency in dams up to 4 ha (Department of Natural Resources and Mines 2003). As with pool covers, there are issues with oxygen transfer and shading.
Aquacap domes are under research at Royal Melbourne Institute of Technology. The
modules (domes) float on the water surface and prevent evaporation by minimising exposure of the water to sun and wind. Trials suggest efficiency of between 70 and 90 per cent
depending on evaporative demand. Domes will have similar issues as E-VapCap in terms of oxygen transfer and shading. Though not yet fully commercialised, cost is estimated to be around $170 000 per hectare (Department of Natural Resources and Mines 2003), which may drop as the product becomes more widely available.
Another type of cover is a water bladder (see Figure 10), which holds water similarly to a wine bladder (Department of Natural Resources and Mines 2003). This system encapsulates the water, eliminating all evaporation, leakage and groundwater intrusion, and keeping the supply free of contaminants. As with the air bubble blanket covers, oxygen transfer is also eliminated, and similar issues (dissolved iron and no habitat provision) are expected. There is no provision for rainfall falling on the cover to enter, and a collection and piping system may be required (Alan Liebeck pers. comm.). The cost is dependent on supplier, and membrane material (high or low density poly ethylene), but is expected to be around
$150 000 to $300 000 per hectare. At this price, its application is likely to be limited to high value storages such as town water supplies.
Figure 10 Water bladder storage in a dam in South Australia (Photo: Alan Liebeck).
The shape of the dam can have a significant impact on the amount of evaporation loss (Stanton 2005). Deep dams with little surface area lose a smaller proportion of water to evaporation, while shallow dams with a large surface area lose a higher proportion. Shallow dams also heat up faster than deep ones, and this increases the potential for evaporation.
This fact also points to the importance of dam maintenance, as silt build-up in the bottom of dams reduces the ‘useful depth’ while decreasing the volume to surface area ratio of the dam. Circular (turkey nest) dams have a very good volume to surface area ratio, and lose proportionately less water than a square dam of similar dimensions (Laing 1981).
Flushing dams
Once salt water has entered a dam, it does not necessarily mean that the quality of all the water in the dam will be compromised. Salt water is ‘heavier’ than fresh water and will naturally settle to the bottom of a dam. This phenomenon is called ‘stratification’ and can result in distinct layers of fresh water sitting on top of salty ones. It provides an opportunity to flush the salty water from the bottom of the dam while retaining the fresh water at the
surface.
Water that is turbulent or regularly mixed does not readily stratify (Turner & Erskine 2005).
Dams which are shallow (less than 2 m deep), and/or where stored water is turned over regularly, are unlikely to develop stratified layers. The rapid heating and cooling of water in dams exposed to sunlight can also increase the potential for mixing (shading of the dam surface can help to reduce this). There is a greater likelihood of separation occurring in dams with greater depth and long detention times. Wellington Dam on the Collie River is one example of a deep dam where flushing or scouring is used to manage salt levels (Mauger et al. 2001). Scouring is also a well-established practice in irrigation dams around Manjimup and Donnybrook.
Most of the irrigation dams in which this technique is employed have pipes installed in the bottom of the dam during the construction phase, typically from the low point in the dam, through the wall of the dam to a discharge point below the dam. Flow may be driven by gravity or pressure head if there is enough fall between the pipe inlet and outlet, with the discharge being controlled by a valve or tap.
A number of these dams have a venturi system set up where water flowing down the dam overflow flows across the outlet of the pipe, creating a vacuum. The vacuum is then used to draw water against gravity from the bottom of the dam. The advantage of the venturi system is that the dam is flushed automatically when it is at full capacity and flushing stops once the dam stops overflowing. This system will not work if the dam overflows only infrequently; in this case a siphon or a back-up pipe through the dam wall may be needed if the dam is likely to need flushing more often than it overflows. It is important that the pipe outlet is not too far below the high-water level in the dam, otherwise too much water may be siphoned from the dam.
Some consideration should be given to siltation when pipe inlet height is selected. Lewis (2002) recommends raising the inlet by 500 mm to allow for sediment build-up and using a strainer to reduce the likelihood of pipe blockage.
It is important to follow appropriate design rules when constructing pipe work through dam walls. Lewis (2002) highlights the importance of cut-off collars, appropriate compaction techniques and pipeline capacity, with pipelines going through embankments. If care is not taken, seepage around the pipeline may lead to tunnelling and dam failure.
If a flushing system is to be installed retrospectively, the options may be limited to a
siphoning system. Many of the basic principles that hold for siphons on bores, as discussed by Seymour (2001), also hold for siphons from dams. The pipe needs to be free of gas or air and the discharge needs to be lower than the inlet for the siphon to work. The need for continuous discharge or siphoning is not as important for dam flushing as it is for bores, and pipe diameter is not as important. Siphons can be primed by filling them with a firefighting pump from the outlet end until the bubbles stop flowing to the surface of the dam. When the firefighting pump is removed, the pipe will begin siphoning from the dam. To stop siphoning, the inlet end can be lifted clear of the water, or a valve at the apex (high point) of the siphon can be opened to let air into the pipe.
As with the piping through dam walls, care needs to be taken to avoid blocking the siphon intake with silt or debris. One method (used with flexible piping) is to tie a float to the top of the inlet and attach weights 50 mm along the cord to keep the inlet suspended 50 mm above the silt in the dam. The depth of the dam at the inlet should be checked to allow an
appropriate length of cord to be run from the pipe inlet to the float. Again, some sort of strainer or filter should be used at the inlet to prevent blockage by rubbish.
To gauge the appropriateness of this technique the level of stratification in the dam and the approximate depths of the different layers need to be determined. Taking water samples at different depths while taking care not to disturb the water column is one way to test for stratification.
Timing is important. Water is usually at its greatest level of stratification at the end of summer, when the dam has been undisturbed by inflow since the end of the winter flows.
This usually coincides with the end of the irrigation season, and the risk of not having enough water to meet the irrigation schedule is minimised.
Care should be taken when disposing of the effluent. The saline water may also contain elevated concentrations of dissolved iron, aluminium, manganese and/or sulphur (Turner &
Erskine 2005). Since water stored in a dam is considered surface water, there is no legal requirement to submit notice of intent to pump or drain with the Commissioner for Soil and Land Conservation. Land managers are however required to ensure that the practice of dam flushing does not result in land degradation, or environmental harm.